The known state of the art includes many different designs of gravimetric measuring instruments. Many of the known designs, also referred to as balances, consist essentially of a force-measuring cell, a load receiver that is operationally connected to the force-measuring cell, a parallel-guiding mechanism constraining the load receiver in vertically guided linear movement, an electronic section to process the weighing signal, and an indicator unit.
The known state of the art includes a variety of functional principles of force-measuring cells or measurement transducers such as for example weighing cells with strain gauges, weighing cells with oscillating strings, or weighing cells based on electromagnetic force compensation (EMFC).
In EMFC weighing cells, the weight of the load is transmitted either directly or by way of one or more force-transmitting levers to an electromechanical measurement transducer which delivers a signal corresponding to the weighing load. The signal is further processed by an electronic portion of the weighing instrument and the result is presented on a display.
Weighing cells with a strain transducer contain a deformable body which is provided with strain gauges. Placing the load on the weighing cell causes an elastic deformation of the deformable body. In many cases, the deformable body is configured as a parallelogram-shaped measuring element (parallel-guiding mechanism with strain gauges), whereby defined zones of deformation or bending zones are created where the strain gauges are arranged. As a result of the load placed on the movable parallel leg, the strain gauges are subjected to tension or compression which causes a change of their electrical resistance in comparison to the stress-free state, and the resistance change represents a measure for the applied load.
In force-measuring cells based on string-oscillators, the mechanical design structure is largely analogous to force-measuring cells based on electromagnetic force compensation or strain gauges, except that an oscillating-string transducer is used in place of an electromagnetic measurement transducer or a strain gauge transducer. As a result of the load, the tension in an oscillating string is increased, and the frequency change, in turn, represents a measure for the applied load.
As mentioned above, force-measuring cells of these types are used in a variety of gravimetric measuring instruments, such as for example balances, gravimetric moisture-determination instruments, weighing modules and the like.
Weighing modules essentially are balances of a kind in which the indicator unit is arranged in a separate place from the balance, for example in an installation with a central display unit for several weighing modules. Weighing modules are used with preference in automated production- and testing systems where a plurality of such weighing modules are united in a compound system of compact dimensions.
A weighing module of this kind has essentially a design structure as shown for example in FIG. 1 of EP 1 726 926 A1. The module has a stationary parallel leg of an approximately cubic shape which at the same time forms the stationary frame in the center of which the movable parallel leg—in this case a vertically movable rod which carries a weighing pan at the top—is constrained in vertically guided movement by diaphragm springs that are arranged, respectively, near the top and bottom of the stationary parallel leg.
However, this strikingly simple concept still has the drawback that it lacks the capability to adjust the parallelism of the diaphragm springs that guide the vertical movement. As is known in the field of weighing, deviations from parallelism between the guide elements of a parallel-guiding mechanism in a balance cause so-called corner load errors, i.e. weighing errors which occur when a weighing load is placed out of center on the load receiver.
The corner load error or, stated in positive terms, the corner load accuracy is a fundamental property of balances with a parallel-guided load receiver. Although one might attempt to achieve a desired degree of corner load accuracy through higher precision in the manufacturing process, this increases on one hand the manufacturing costs of balances and weighing modules, while on the other hand even the highest achievable accuracy of a machine tool falls short of attaining the corner load accuracy of the order of about 1/50,000 to 1/1,000,000 of the weighing capacity.
For this reason, one uses a fundamentally different approach with high-precision balances, in that on the one hand a relatively wide tolerance in the machining accuracy of the relevant components is accepted from the outset, while on the other hand adjustment possibilities are provided at the critical points of the parallel-guiding mechanisms, whereby the parallelism of the guide members can be adjusted to the level that meets the required corner load accuracy. This adjustment of the corner load accuracy is performed following the assembly phase in the now operational gravimetric measuring instrument, using an automatic or manual inspection and adjustment process which may include additional settings, for example for the linearity adjustment and the span calibration.
In a parallelogram-shaped measuring element, i.e. a parallel-guiding mechanism which constrains the weighing pan carrier in a parallel movement by means of two parallel, essentially horizontal parallel guides, corner load errors are caused primarily by the fact that the parallel guides deviate slightly from an ideal, absolutely parallel alignment. The relative magnitude of the corner load error, i.e. the ratio between the error of the indicated weight and the amount of the test weight being used corresponds approximately to the relative geometric deviation by which the error is caused. A distinction is made between a corner load error in the lengthwise direction and a corner load error in the transverse direction of the parallel-guiding mechanism, in accordance with the direction in which the test weight is shifted on the weighing pan in the corner load test of the balance. A corner load error in the lengthwise direction occurs when the vertical distance of the parallel guides at the end where they are connected to the stationary parallel leg is not exactly the same as at the opposite end where they are connected to the movable parallel leg. A corner load error in the transverse direction on the other hand occurs when the two parallel guides are twisted relative to each other, i.e. a condition where the distance between the parallel guides varies across the width of the parallel guides.
A corner load adjustment feature of this kind is disclosed for example in U.S. Pat. No. 4,606,421 A1. The stationary parallel leg of the parallel-guiding mechanism disclosed in that reference has elastically deformable bending zones located in different respective planes above each other. The tilt axis of the first bending zone is oriented in the lengthwise direction of the parallel-guiding mechanism, while the tilt axis of the second bending zone is arranged at a right angle to the lengthwise direction of the parallel-guiding mechanism. By means of four adjustment screws, the upper end portion of the stationary parallel leg can be tilt-adjusted in relation to the lower end portion of the stationary parallel leg, whereby the corner load errors in the lengthwise as well as in the transverse direction can be corrected.
As has already been mentioned, weighing modules of the kind named above, for example as described and illustrated in EP 1 726 926 A1, lack the capability for adjusting the parallelism of the diaphragm springs that guide the load receiver, so that only a limited level of corner load accuracy can be achieved in these modules even with precise and thus cost-intensive manufacturing methods.
There is an object to provide a parallel-guiding mechanism, preferably for a compact weighing module, wherein the capability of adjusting the parallelism of the parallel guides is realized through a simple, functionally reliable and cost-effective feature, whereby a specified corner load accuracy can be set by adjustment, so that no narrow and hard-to-meet tolerances have to be imposed on the manufacturing process of a parallel-guiding mechanism.